1913] HEMENWAY—SIEVE-TUBE 237 
dicotyledons is very characteristic for each species and usually 
for each genus. In many cases it is arranged in bands concentric 
with the cambium, as in Acer, Populus, and Crataegus; in some 
cases in groups opposite the large or aggregate rays, as in Alnus, 
Carpinus, and Drimys; in other cases in irregular patches, as in 
Ostrya and Celtis; and finally, in various combinations of these 
arrangements, as in Quercus. 
Plate figs. 1-3 show some of these peculiarities of phloem 
structure. Fig. 1 is a transverse section of Acer macrophyllum; 
the lighter horizontal bands are the hard bast cells; between these 
are the sieve-tubes and parenchyma cells; while the dark vertical 
lines are the phloem rays; in the lower portion of the figure the 
cambium and some xylem are seen. Fig. 2 is a similar view of 
Alnus incana; here we note that the hard bast occurs chiefly 
opposite the aggregate ray. Fig. 3 is a similar view of Quercus 
Garryana; a large group of hard bast is seen above the large ray 
in the lower left portion of the figure, while several smaller groups 
appear here and there in other parts of the phloem. 
Fig. 4 is a transverse section of the stem of Ranunculus fascicu- 
laris; the central, lighter portion of the three bundles shown here 
is phloem. Fig. 5 is a similar view of Chenopodium album, and 
fig. 6, of Amaranthus paniculatus. The darker areas just above 
the groups of large vessels are phloem. This peculiar, scattered 
arrangement of bundles in a woody cylinder would suggest a pos- 
sible point of origin for the monocotyledons. 
Companion cells are rare if not wanting in many of the lower 
dicotyledons. It is probable that parenchyma cells play the part 
of companion cells here. 
The end walls of the ray cells in the phloem of some woody 
dicotyledons show sieve-platelike pitting, but they did not show 
any callus formation. This formation is easily observed in tan- 
gential sections of Phellodendron, Rhus, Ilex, Acer, Gymnocladus, 
Acanthopanax, and many other species. There are often thick- 
walled, unlignified, phloem-parenchyma cells in both woody and 
herbaceous dicotyledons that show lateral sieve-platelike pittings, 
but no terminal sieve-plates or callus were seen. 
In the herbaceous dicotyledons studied, often the most striking 
